The present invention relates to an active anti-vibration apparatus or vibration damping apparatus which mounts precision equipment such as an electron microscope, semiconductor exposure device, and the like, and can remarkably improve the transmission characteristics of vibrations from a setting foundation such as a floor as compared to a conventional apparatus.
The present invention relates to an active anti-vibration apparatus with a floor vibration feedforward function of suppressing vibrations that reach an anti-vibration table which carries a precision equipment via mechanism members of a plurality of damper support legs which support the anti-vibration table.
In general, precision equipment that is vulnerable to vibrations are mounted on an anti-vibration table. For example, an optical microscope, an X-Y stage for exposure, and the like are mounted. Especially, the X-Y stage for exposure must be mounted on the anti-vibration table from which external vibrations are removed as much as possible, so as to realize proper and quick exposure. This is because exposure must be done in a perfectly stand-still state of the X-Y stage for exposure. Furthermore, it is noted that the X-Y stage for exposure has, as its operation mode, intermittent motions called step & repeat motions, and produces repetitive step vibrations, which induce vibrations of the anti-vibration table. When such vibrations remain unsettled, it is impossible to start exposure. Hence, the anti-vibration table is required to realize an anti-vibration function against external vibrations, and a vibration control function of forcible vibrations caused by motions of the mounted equipment itself with a good balance.
In recent years, in place of a step & repeat type semiconductor exposure device that irradiates exposure light onto a silicon wafer mounted on the X-Y stage after the stage perfectly comes to a halt, a scan type semiconductor exposure device that irradiates exposure light onto a silicon wafer while scanning an X-Y stage and the like has also come into existence. Likewise, an anti-vibration table used in such device is required to achieve an anti-vibration function against external vibrations and a vibration control function of forcible vibrations caused by motions of the mounted precision equipment with a good balance.
An anti-vibration method includes active and passive methods.
In one active vibration control method, an adaptive digital filter is arranged to have vibration information of a vibration source as a reference input and vibration information of the object to be vibration-damped as an error input, and is used for implementing feedforward control. Such method is popularly used in suspension control for vehicles and the like. Since this vibration control method uses a digital feedforward compensator, it can automatically derive and realize appropriate characteristics of the compensator by its own adaptive operations in correspondence with the characteristics of an anti-vibration table even when the characteristics of the object to be vibration-damped are unknown. This control method is very effective today since a discrete arithmetic device such as a DSP (digital signal processor) can be easily used. As a vibration control method using the DSP, a "vibration control method and apparatus" disclosed in Japanese Patent Laid-Open No. 6-235439 or the like has been proposed.
FIG. 14 is a diagram showing the arrangement of a general anti-vibration apparatus. An anti-vibration table 1 that carries a precision equipment is set on a setting foundation 3 via support mechanisms 2. A first vibration detector 4 for detecting vibration information in two horizontal directions (X- and Y-directions) and a vertical direction (Z-direction) is arranged on the anti-vibration table 1, and a second vibration detector 5 for similarly detecting vibration information in the three directions is arranged on the setting foundation 3. Also, an actuator 6 for generating vibrating forces in the two horizontal directions and vertical direction is arranged on the anti-vibration table 1.
The three components output from the first vibration detector 4 are input to a parameter estimation mechanism 14 in an adaptive filter 13 via amplifiers 10, band-pass filters 11, and A/D converters 12. Also, the three components output from the second vibration detector 5 are input to the parameter estimation mechanism 14 and feedforward compensators 18 in the adaptive filter 13 via amplifiers 15, band-pass filters 16, and A/D converters 17. The three outputs from the parameter estimation mechanism 14 are connected to the actuator 6 via the feedforward compensators 18, D/A converters 19, mixing circuits 20, and drivers 21. The outputs from the band-pass filters 11 on the first vibration detector 4 side are also input to the mixing circuits 20 via a feedback compensator 22.
The first vibration detector 4 detects vibrations of the anti-vibration table 1, and the amplifiers 10 amplify the obtained detection signals. Thereafter, the band-pass filters 11 pass only components in a necessary frequency band to obtain vibration information. The feedback compensator 22 arithmetically operates the obtained vibration information, and feeds back the arithmetic operation result to the actuator 6 via the mixing circuits 20 and the driver 21. On the other hand, the second vibration detector 5 detects vibrations of the setting foundation 3, and the amplifiers 15 amplify the obtained detection signals. Thereafter, the band-pass filters 16 pass only components in a necessary frequency band to obtain vibration information. The adaptive filter 13 arithmetically operates the obtained vibration information, and inputs obtained manipulated variables to the drivers 21, thus driving the actuator 6.
In the above-mentioned vibration control method, not only the vibrations of the anti-vibration table 1 are detected by the vibration detector 4, and the output signals from the detector 4 are subjected to compensation and fed back to the actuator 6 that applies control forces to the anti-vibration table 1, but also the vibrations of the setting foundation 3 are subjected to appropriate compensation and fed forward to the actuator 6. According to the above-mentioned vibration control method, the insulating performance of vibrations from the setting foundation 3 can be greatly improved, and the vibration standard of the setting foundation 3 is relaxed.
As an example of the feedforward compensator, for example, as in a "floor vibration disturbance control method for an anti-vibration table" disclosed in Japanese Patent Laid-Open No. 5-263868, some characteristics of an active anti-vibration apparatus including those of an actuator are actually measured to implement a compensator. The reference discloses a method of deriving optimal characteristics of a feedforward compensator on the basis of the shake test response of the anti-vibration table using the actuator of the active anti-vibration apparatus and the vibration transmittance from the setting foundation to the anti-vibration table.
However, in the above-mentioned anti-vibration apparatus, interference components are produced and superposed on vibrations in the two horizontal directions and vertical direction depending on the attachment and center of gravity of the actuator 6. For this reason, when the convergence rates of the estimated parameters of the anti-vibration table 1 in the individual directions vary in units of directions, the transition of a given estimated parameter may influence parameter estimation in other directions. As a consequence, the parameters do not converge or diverge, thus deteriorating the anti-vibration performance.
In the above-mentioned active anti-vibration apparatus, the actuator 6 generates control forces to cancel vibrations in the two horizontal directions and vertical direction. However, depending on the attachment position of the actuator 6 and the center of gravity of the anti-vibration apparatus, interference components in other directions are produced, thus deteriorating the anti-vibration performance.
As described above, the anti-vibration table is classified into passive and active tables. In recent years, in order to meet requirements of high-precision positioning, high-precision scanning, high-speed movement, and the like for an equipment mounted on the anti-vibration table, active anti-vibration apparatuses have proliferated. As an actuator used in such apparatus, a pneumatic spring, voice coil motor, piezoelectric element, and the like are known. An active anti-vibration apparatus using a pneumatic spring as an actuator will be explained in detail below.
FIG. 15 shows a damper support leg using a pneumatic spring as an actuator. In FIG. 15, reference numeral 101 denotes an anti-vibration table that carries a precision equipment such as an X-Y stage and the like; and 102, a fastening plate for fastening a damper support leg 103 to the anti-vibration table 101. The internal mechanism of the damper support leg 103 is as follows. Reference numeral 104V (H) denotes a vertical (horizontal) driving pneumatic spring; 105V (H), a vertical (horizontal) vibration sensor; 106V (H), a vertical (horizontal) position sensor; 107V (H), a vertical (horizontal) servo valve; 108, laminated rubber; 109, a horizontal pilot pressure mechanical spring; and 110, a casing. Note that the vibration sensor normally comprises an acceleration sensor.
The arrangement of a feedback device for the damper support leg 103 with the above-mentioned mechanism members will be explained below. Reference numeral 111 denotes a filter circuit for converting signals output from the vibration sensors 105V and 105H into electrical signals and appropriately filtering the electrical signals; 112, a voltage-current (VI) converter 112 for supplying currents to the servo valves 107V and 107H; 113, a position detector for converting the outputs from the position sensors 106V and 106H into electrical signals; 114, a target voltage setting unit; 115, a error amplifier; and 116, a PI compensator. These elements make up a feedback device 117. Note that P means proportional, and I integral. FIG. 15 illustrates only the feedback device 117 for the horizontal direction, but a feedback device for the vertical direction has the same arrangement. Furthermore, in general, the anti-vibration table 101 is supported using a plurality of damper support legs 103, and feedback devices 117 are arranged in correspondence with the vertical and horizontal directions of each damper support leg.
In addition to the above-mentioned loop arrangement which comprises the feedback devices 117 in correspondence with the plurality of damper support legs 103, an arrangement disclosed in Japanese Patent Laid-Open No. 7-83276 is known. This reference discloses the arrangement of a control loop based on general motions, i.e., translation and rotation, of the Adanti-vibration table 101. Since servo adjustment can be done in units of motion modes such as translation and rotation, the position control of the anti-vibration table can be made precisely, and consequently, the performance of a precision equipment mounted on the anti-vibration table 101 can be fully utilized. Japanese Patent Application No. 8-19238 ("active anti-vibration apparatus and active anti-vibration method") discloses an apparatus arrangement which adopts, as a basic control arrangement, closed loops in units of motion modes, and performs so-called "floor vibration feedforward compensation in units of motion modes". In this compensation, a plurality of vibration sensors are set on a floor, floor vibrations are extracted in units of motion modes from these sensor outputs, and signals obtained by performing appropriate compensation of these extracted floor vibrations are fed forward.
Conveniently, floor vibration feedforward adopts loop structures in units of motion modes in correspondence with the frameworks of the closed loops, which support the anti-vibration table in a stable position, in units of motion modes, and such apparatus arrangement has an excellent feature that the anti-vibration ratios can be adjusted in units of motion modes without any interferences. In a conventional method, floor vibrations are represented by the measurement value of a vibration sensor arranged at one position, and this output is subjected to appropriate compensation and fed forward to actuators of the individual axes. However, the above-mentioned apparatus arrangement that feeds forward floor vibrations in units of motion modes has greatly superior anti-vibration ratio characteristics to the conventional method.
However, in order to further improve the anti-vibration ratio, the fact that floor vibrations enter the vibration table that carries a precision equipment via the damper support legs as routes must be taken into consideration. Vibrations that reach the anti-vibration table are traced back to floor vibrations. To suppress transmitted vibrations, vibrations concentrated at the foot of each damper support leg must be considered. That is, floor vibrations are transmitted to a device setting foundation member which fixes each damper support leg to vibrate it, and the vibrations enter the damper support leg of each axis from its foot, thus vibrating the anti-vibration table which carries a precision equipment.
If the device setting foundation member is rigid and the member is assumed to be integrated with a floor in a required anti-vibration frequency band, identical vibrations can be detected independently of the attachment position of the vibration sensor on the floor or on the device setting foundation member. However, in practice, since the device setting foundation member itself is not rigid and the contact state between the member and the floor is not uniform depending on its position, the vibration level when floor vibrations have traveled to the device setting foundation member normally becomes larger than that of the floor itself. On the other hand, when the mounted precision equipment is accelerated/decelerated, each damper support leg generates a driving force against a counterforce to apply a vibrating force to the device setting foundation member at its foot. With this force, the member which is not rigid statically deforms, or in many cases, local vibrations are produced. The trouble is that such local vibrations vibrate the anti-vibration table via the corresponding damper support leg again. Since these vibrations are local, the vibration entry state varies in units of damper support legs.
FIG. 16 shows the arrangement of a conventional semiconductor exposure device that adopts floor vibration feedforward compensation. In FIG. 16, the anti-vibration table 101 that carries an X-Y stage 118 as a precision equipment is rigidly connected to the damper support legs 103 via the fastening plates 102, and is supported by a device setting foundation member 119. Reference numeral 120 denotes leveling mechanisms for adjusting the height and inclination of the entire apparatus on the device setting foundation member 119.
In general, vibration sensors 122x, 122y, and 122z for detecting vibrations in the vertical axis and the two horizontal axes are set on, e.g., a single central portion of the device setting foundation member 119 so as to execute floor vibration feedforward control for suppressing transmission of vibrations on a floor 121 to the anti-vibration table 101. Alternatively, vibration sensors 122 as many as the required number of axes are arranged on an appropriate single portion on the floor 121. Note that x, y, and z indicate the coordinate axes in FIG. 16, which agree with the vibration measurement axes. That is, the outputs from the vibration sensors arranged on a single portion on the device setting foundation member 119 or floor 121 represent vibrations transmitted to the anti-vibration table 101. Signals obtained by appropriate compensation of these outputs are fed forward to actuators in the plurality of damper support legs 103 that support the anti-vibration table 101, thereby improving the anti-vibration ratio. The vibration measurements of the vibration sensors 122x, 122y, and 122z are done under the assumption that vibrations on the floor 121 and the device setting foundation member 119 as vibration sources remain the same independently of positions.
However, in practice, attenuation of vibrations over distance vary depending on the position of a vibrating source such as a power machine or the like with respect to the apparatus shown in FIG. 16 and, hence, the vibration state also differs depending on the position on the floor 121. Since the leveling mechanisms 120 and the device setting foundation member 119 are not rigid, the members 119, 120, and 121 do not vibrate integrally in fact upon directly receiving vibrations of the floor 121. Furthermore, it is also noted that the damper support legs 103 strongly kick the device setting foundation member 119 since the anti-vibration table is maintained at a predetermined position against a counterforce produced upon accelerating/decelerating the X-Y stage 118. At this time, the device setting foundation member 119 which is not rigid statically distorts, and dynamically excites local vibrations near the feet of the damper support legs 103.
In consideration of the above-mentioned phenomenon, in the conventional floor vibration feedforward compensation using vibration measurement signals obtained at a single position on the device setting foundation member 119 or floor 121, vibrations which are transmitted from the feet of the damper support legs to the anti-vibration table 101 cannot be removed more effectively. Also, an apparatus arrangement that extracts the motion mode of floor vibrations using a plurality of vibration sensors arranged on the floor and feeds forward the extracted motion mode has been proposed. However, such arrangement cannot adequately suppress local vibrations of the device setting foundation member, which are produced upon acceleration or deceleration of the precision equipment mounted on the anti-vibration table and are transmitted via the damper support legs.
The transmission routes of floor vibrations to the anti-vibration table that carries the precision equipment include mechanism members that make up the damper support legs of the individual axes. Furthermore, normally, each damper support leg is placed on the device setting foundation member. That is, since the mechanical vibration mode of the device setting foundation member is also present, a marked improvement of anti-vibration performance cannot be expected even when floor vibrations are measured at a single position on the floor or device setting foundation member and are fed forward to actuators of the individual axes via appropriate compensators as in the prior art.